Polarizing plate, method for manufacturing same and liquid-crystal display device comprising same

ABSTRACT

The present invention relates to a polarizing plate including a polarizer and a polyester film formed on an upper side of the polarizer, wherein the polyester film has a maximum thermal shrink angle of about 10° or less, and any one of a refractive index of x-axis direction nx at a wavelength of 550 nm and a refractive index of y-axis direction ny at a wavelength of 550 nm of about 1.65 or more; a method of preparing the polarizing plate; and a liquid crystal display apparatus comprising the polarizing plate.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a national stage and claims priority to and thebenefit of International Application No. PCT/KR2014/007487 filed on Aug.12, 2014, which claims priority to and the benefit of Korean PatentApplication No. 10-2013-0103080 filed Aug. 29, 2013 and Korean PatentApplication No. 10-2014-0087803 filed Jul. 11, 2014, the entire contentsof which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a polarizing plate, a method ofpreparing the same, and an optical display apparatus including the same.

BACKGROUND ART

Polarizing plates are used inside and outside a liquid crystal cell forpurpose of controlling an oscillation direction of light in order tovisualize display patterns of a liquid crystal display apparatus. Thepolarizing plate includes a polarizer, and a protective film, which isformed on at least one side of the polarizer. Typically, although theprotective film is a triacetyl cellulose (TAC) film, the TAC film ismore expensive than typical polymer films. Therefore, low-priced polymerfilms including polyethylene terephthalate (PET) films are used toreplace the TAC film.

A common PET film is a film which is drawn in a machine direction (md)and/or a transverse direction (td) by a certain draw ratio in order toimprove a yield and have a range of phase difference (i.e.,retardation). However, the common pet film which is drawn in md and tdhas high molecular orientation angle, and thus there is a largedeviation between an absorption axis of a polarizer and an optical axisof the pet film. Therefore, polarization degree of the polarizing plateand brightness of a screen may be decreased. Further, due to this,contrast ratio (cr) of the liquid crystal display apparatus may bedeteriorated. Still further, the common PET film may decreasepolarization degree of the polarizer and transmittance of the polarizingplate and make rainbow spots worse, when exposed at high temperature fora long period of time. Moreover, when the PET film is drawn, a portionof, particularly, an end portion of the PET film may suffer fromdeterioration in optical properties due to its asymmetric molecularorientation in incorporating into a polarizing plate, and thus, therehas been a limitation in its use. Further, since the PET film is a drawnfilm, the film can suffer from rainbow spots in using it in liquidcrystal display apparatus.

The background art of the present invention is described in KR10-2011-0014515 A.

DISCLOSURE Technical Problem

It is one object of the present invention to provide a polarizing platethat can suppress rainbow spots, secure viewing angle, improve an imagequality.

It is another object of the present invention to provide a polarizingplate that can improve polarization degree of the polarizer andtransmittance, thereby having excellent optical properties and highcontrast ratio.

It is a further object of the present invention to provide a polarizingplate that may be no deterioration of polarization degree thereof, or beable to minimize the deterioration, and thus the polarizing plate cansuppress rainbow spots, even if the polarizing plate is exposed to hightemperature.

It is a further object of the present invention to provide a polarizingplate having high economic feasibility by using the drawn polyester filmas a protective film for the polarizing plate over the wholeorientations.

Technical Solution

An aspect of the subject invention relates to a polarizing platecomprising a polarizer; and a polyester film formed on an upper side ofthe polarizer, wherein the polyester film has a maximum thermal shrinkangle of about 10° or less, and any one of a refractive index of x-axisdirection (nx) at a wavelength of 550 nm and a refractive index ofy-axis direction (ny) at a wavelength of 550 nm of about 1.65 or more.

Another aspect of the subject invention relates to a polarizing platecomprising a polarizer; and a polyester film formed on an upper side ofthe polarizer, wherein the polyester film has an absolute value ofmolecular orientation angle (θr) of about 5° or less.

In a specific example, an absolute value of nx−ny may be about 0.1 toabout 0.2.

In a specific example, the polyester film may have a maximum thermalshrinkage of about 0.8% or less.

In a specific example, the polyester film may have an absolute value ofmolecular orientation angle (θr) of a polyester molecule based on atransverse direction (TD) of about 0° to about 5°.

In a specific example, the polyester film may have a thickness of about25 to about 115 μm, and a front retardation (Ro) of about 5,000 to about15,000 nm at a wavelength of 550 nm.

In a specific example, the front retardation (Ro) may be about 10,100 toabout 12,000 nm.

In a specific example, the polyester film may be a TD drawn film.

In a specific example, the polyester film may have a degree ofbiaxiality (NZ) of about 1.8 or less at a wavelength of 550 nm, asrepresented by Equation 1:NZ=(nx−nz)/(nx−ny)  Equation 1

(whererin nx, ny and nz are refractive index in x-axis direction, y-axisdirection and z-axis direction of the polyester film at a wavelength of550 nm, respectively).

In a specific example, the polyester film may have a retardation inthickness direction (Rth) of about 15,000 nm or less at a wavelength of550 nm, as measured by Equation 2:Rth={(nx+ny)/2−nz}×d  Equation 2

(whererin nx, ny and nz are refractive index in x-axis direction, y-axisdirection and z-axis direction of the polyester film at a wavelength of550 nm, respectively; and d is a thickness of the polyester film (unit:nm)).

In a specific example, the polyester film may be a film formed of atleast one of polyethylene terephthalate, polybutylene terephthalate,polyethylene naphthalate and polybutylene naphthalate.

In a specific example, an optical film may be further formed to a lowerside of the polarizer.

In a specific example, the optical film may have a front retardation(Ro) of about 40 to about 60 nm at a wavelength of 550 nm.

In a specific example, the optical film may be a film formed of at leastone of cellulose, polyester, cyclic polyolefin, polycarbonate, polyethersulfone, polysulfone, polyamide, polyimide, polyolefin, polyacrylate,polyvinyl alcohol, polyvinyl chloride, and polyvinylidene chlorideresins.

In a specific example, the polarizing plate may have a polarizationdegree of about 99.99% or more, and a transmittance of about 40% ormore.

Another aspect of the subject invention relates to a method of preparinga polarizing plate, comprising drawing a melt-extruded polyester resinby a draw ratio of about 2 to about 10 in TD only, andthermal-stabilizing the drawn polyester resin at about 100 to about 300□to prepare a polyester film; and bonding the polyester film to one sideof a polarizer.

In a specific example, the method may further comprise bonding anoptical film to the other side of the polarizer.

Another aspect of the subject invention relates to a liquid crystaldisplay apparatus comprising the polarizing plate.

Advantageous Effects

The present invention provides a polarizing plate, a method of preparingthe same, and an optical display apparatus including the same that cansuppress rainbow spots, secure viewing angle, improve an image quality,and improve polarization degree of the polarizer and transmittance,thereby having excellent optical properties and high contrast ratio.Further, the present invention provides a polarizing plate, a method ofpreparing the same, and an optical display apparatus including the samethat may be no deterioration of polarization degree thereof, or able tominimize the deterioration, and thus the polarizing plate can suppressrainbow spots, even if the polarizing plate is exposed to hightemperature. More further, the present invention provides a polarizingplate, a method of preparing the same, and an optical display apparatusincluding the same having high economic feasibility by using the drawnpolyester film as a protective film for the polarizing plate over thewhole orientations.

DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a polarizing plate according to oneembodiment of the present invention.

FIG. 2 is a sectional view of a polarizing plate according to anotherembodiment of the present invention.

FIG. 3 is a sectional view of a liquid crystal display apparatusaccording to one embodiment of the present invention.

FIG. 4 is a conceptual view of thermal shrinkage in the presentinvention.

BEST MODE

Certain embodiments of the present invention will now be described inmore detail such that they can easily have been made by an ordinaryskilled person in the art to which this invention pertains withreference to the accompanying drawings. The present invention may beembodied in different ways and is not limited to the followingembodiments. In the drawings, elements irrelevant to the description ofembodiments of the present invention will be omitted for clarity. Thesame reference numerals will be used throughout the drawings and thedescription to refer to the same or similar constitutional elements.

As used herein, the terms such as “upper side” and “lower side” aredefined with reference to the accompanying drawings. Thus, it will beunderstood that the term “upper side” can be used interchangeably withthe term “lower side” or vice versa when viewed from different angles.

As used herein, the term ‘thermal shrinkage’, referring to FIG. 4, maybe a value obtained from |B−A|/A×100, wherein, in a square polyesterfilm specimen 10 having an about 200 mm length in MD and an about 200 mmlength in TD, it draws a circle 10 a having a radius A (0<A≤150 mm,e.g., 100 mm) based on the center point of the specimen 10, equallydivides a circumference of the circle 10 a into a plurality of segmentat the same angle α (0<α≤10°, specifically about 5° in a certain rangefrom the radius 10 b of the circle parallel to MD to further draw aplurality of lines 10 c of connecting one point on the circumference andthe center of the circle, and measures a length B of the line 10 c afterthe specimen 10 or circle 10 a was left in any one condition selectedfrom i) at about 85° C. for about 30 minutes, ii) at about 100° C. forabout 30 minutes, iii) at about 120° C. for about 30 minutes, and iv) atabout 150° C. for about 30 minutes. As used herein, the term ‘maximumthermal shrinkage’ means the maximum value among the measured thermalshrinkages.

As used herein, the term ‘maximum thermal shrink angle’ means to anangle between a radius 10 b of the circle parallel to a MD and astraight line indicating the maximum thermal shrinkage when plotting anangle α in FIG. 4 and the thermal shrinkages measured in FIG. 4 to eachother according to one point on the circumference equally divided alongthe angle α using the polyester film specimen 10. As a result ofplotting, the plotting result having a peanut form may be obtained.

As used herein, ‘nx’, ‘ny’ and ‘nz’ are refractive index inthree-dimensional coordinate system (x-axis direction, y-axis direction,and z-axis direction (thickness direction)) of the polyester film at awavelength of 550 nm, respectively, unless specifically mentioned. Forexample, ‘x-axis direction’ may be ‘MD’, or ‘y-axis direction’ may be‘TD’.

Hereinafter, a polarizing plate according to one embodiment of thepresent invention will be described in detail with reference to FIG. 1.

Referring to FIG. 1, a polarizing plate 100 according to one embodimentof the invention may include a polarizer 110, and a polyester film 120formed on an upper side of the polarizer 110, wherein the polyester film120 may have a maximum thermal shrink angle of about 10° or less, andany one of nx and ny of about 1.65 or more.

A polarizing plate in a liquid crystal display apparatus may be exposedto high temperature for a long period of time. In this case,polarization degree of the polarizing plate may be decreased, therebydeteriorated optical properties of the liquid crystal display apparatus.The polarizing plate according to one embodiment of the presentinvention may include the polyester film 120 having the maximum thermalshrink angle of about 10° or less, thus even if the polarizing plate isexposed to high temperature, polarization degree and transmittancethereof may not be decreased. The maximum thermal shrink angle of thepolyester film may be specifically about 0° to about 10°, morespecifically about 0° to about 9°, further more specifically about 0° toabout 7°, for example 0°, 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, or 10°,and further preferred closer to 0°.

Generally, conventional polyester film has crystallization behaviorduring drawing process. Thus, a polarizing plate using the polyesterfilm may suffer from rainbow spots, thereby deteriorating the imagequality. If the polyester film has both of nx and ny of less than about1.65, or both of nx and ny of about 1.65 or more, the polarizer havingthe polyester film as a protective film may suffer from rainbow spots bybirefringence due to change of retardation depending on incident angleand wavelength. However, according to an embodiment of the presentinvention, the polyester film 120 in the polarizing plate has any one ofnx and ny of about 1.65 or more, so that it can remarkably suppressrainbow spots.

In one embodiment, nx may be about 1.65 or more, specifically about 1.67to about 1.75, and ny may be less than about 1.65, specifically about1.45 to about 1.60. In another specific example, ny may be about 1.65 ormore, specifically about 1.67 to about 1.72, more specifically about1.69 to about 1.72, and nx may be less than about 1.65, specificallyabout 1.45 to about 1.60. Herein, an absolute value of nx−ny (|nx−ny|)may be about 0.1 to about 0.2, specifically about 0.1 to about 0.18, forexample 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, or 0.18. Thereby,the polyester film may improve viewing angle and also suppress rainbowspots.

The polyester film 120 may have a maximum thermal shrinkage of about0.8% or less, specifically about 0 to about 0.8%, more specificallyabout 0 to about 0.6%, for example 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%,0.6%, 0.7%, or 0.8%, and more preferred closer to 0%. Within this range,there may be no problem of deterioration of optical properties of aliquid crystal display apparatus due to decrease in transmittance of apolarizing plate when the polarizing plate is exposed to hightemperature for a long period of time. In one embodiment, the polyesterfilm may have thermal shrinkage of about 0 to about 0.8% over the wholeorientations (about 0° to about 360°) based on a circle of FIG. 4.Within this range, the polyester film may have improved opticalproperties.

The polyester film 120 has a super high retardation due to drawing it byhigh draw ratio, so that it can suppress rainbow spots to prevent thepolarizing plate from deteriorating an image quality when incorporatedthe polarizing plate into the liquid crystal display apparatus. In aspecific example, the polyester film 120 may have a thickness of about25 to about 115 μm and front retardation (Ro) of about 5,000 nm to about15,000 nm, specifically about 10, 100 to about 12,000 nm at a wavelengthof 550 nm. Within the range, when the polyester film is used as aprotective film of a polarizer, it can suppress rainbow spots. Moreover,it can inhibit a light leakage phenomenon which leaks light from sides,and further suppress a change of retardation depending on an incidentangle to thereby preventing the difference of retardations fromincreasing.

The polyester film 120 may have a degree of biaxiality (NZ) of about 1.8or less, specifically, about 1.0 to about 1.8 at a wavelength of 550 nm,as represented by Equation 1. Within this range, the polyester film canhave the effect of controlling rainbow spots due to birefringence:NZ=(nx−nz)/(nx−ny)  Equation 1

(whererin nx, ny and nz are refractive index in x-axis direction, y-axisdirection and z-axis direction of the polyester film at a wavelength of550 nm, respectively).

The polyester film 120 may have a retardation in thickness direction(Rth) of about 15,000 nm or less, for example about 10,000 to about13,000 nm at a wavelength of 550 nm, as measured by Equation 2. Withinthis range, the polyester film can have the effect of controllingrainbow spots due to birefringence:Rth={(nx+ny)/2nz}×d  Equation 2

(whererin nx, ny and nz are refractive index in x-axis direction, y-axisdirection and z-axis direction of the polyester film at a wavelength of550 nm, respectively, and d is a thickness of the film (unit: nm)).

The polyester film 120 may have an absolute value of molecularorientation angle (θr) based on TD in the polyester molecular of about5° or less, specifically about 0 to about 5°, for example 0°, 1°, 2°,3°, 4° or 5°. Within this range, it can improve polarization degree ofthe polarizing plate and brightness of a screen, thereby increasingcontrast ratio thereof, and further preventing polarization degree ofthe polarizing plate from deteriorating even if exposed to hightemperature for a long period of time. The molecular orientation anglemay be measured by any typical methods, for example, by usingKOBRA-WX100 (Oji Co., Ltd) and AXOSCAN (Axometrics Co., Ltd).

The polyester film 120 may be any transparent films formed of apolyester resin without limitation. In embodiments, the polyester filmmay be a film formed of at least one of polyethylene terephthalateresin, polybutylene terephthalate resin, polyethylene naphthalate resin,and polybutylene naphthalate resin.

The polyester film 120 may have a thickness of about 25 μm to about 115μm. Within this range, when the film is stacked on the polarizer, theobtained product can be used as a polarizing plate.

Although not shown in FIG. 1, the polyester film 120 may include afunctional coating layer on an upper side thereof, for example, a hardcoating layer, an anti-reflective layer or an anti-fingerprint layer toimpart functionality. The functional coating layer may have a thicknessfrom about 1 μm to about 10 μm. Within this range, when the film isstacked on the polarizer, the obtained product can be used as apolarizing plate.

In addition, although not shown in FIG. 1, the polyester film 120 mayfurther include a side coating layer on a lower side thereof. Thepolyester film has a hydrophobic surface. In particular, when apolyethylene terephthalte film is used as a protective film, theprotective film exhibits higher hydrophobicity. To apply such a film tothe polarizing plate, the film is subjected to surface modification toconvert a surface thereof from a hydrophobic surface to a hydrophilicsurface. When surface modification using sodium hydroxide, which is usedin existing cellulose films, is used for surface modification of theprotective film, the surface of the protective film can beinsufficiently modified or can be damaged. To solve such problems, asurface coating layer including a highly adhesive primer havinghydrophobic and hydrophilic functional groups may be formed on theprotective film. The primer having hydrophobic and hydrophilicfunctional groups may include polyester resins, polyvinyl acetateresins, or mixtures thereof, without being limited thereto. Mechanicalproperties and low water vapor permeability of the protective film maybe maximized through addition of the surface coating layer, therebyimparting high resistance to severe external conditions to thepolarizing plate. In addition, the surface coating layer may be formedbetween the protective film and the polarizing plate to improve adhesionbetween the protective film and the polarizer.

Since the polarizer 110 has molecules aligned in a specific direction,the polarizer transmits only light of a specific direction whenincorporated into the liquid crystal display apparatus. The polarizermay be prepared by dyeing a polyvinyl alcohol film with iodine or adichroic dye, followed by drawing it in a certain direction.Specifically, the polarizer is prepared through processes of swelling,dyeing, drawing, and cross-linking. Each process may be performed by amethod generally known to those skilled in the art.

The polarizer 110 may have a thickness from about 5 μm to about 30 μm.Within this range, the polarizer can be used to the polarizing plate fora liquid crystal display apparatus.

Also, the polarizing plate 100 may have a thickness from about 25 μm toabout 500 μm. Within this range, the polarizing plate can be used as apolarizing plate for a liquid crystal display apparatus. The polarizingplate may have a polarization degree of about 99.99% or more, forexample about 99.99 to about 99.999%. Further, the polarizing plate mayhave transmittance (for example, as measured at visible light range,i.e., a wavelength of 550 nm) of about 40% or more, for example about 40to about 80%. Within this range, when the polarizing plate isincorporated into the liquid crystal display apparatus, the liquidcrystal display apparatus may not suffer from deterioration in opticalproperties.

Although not shown in FIG. 1, an adhesive layer for the polarizing platemay be formed between the polarizer 110 and the polyester film 120 toimprove mechanical strength of the polarizing plate. The adhesive layermay include any typical adhesives, for example, at least one ofwater-based adhesives, pressure-sensitive adhesives, and photocurableadhesive. In addition, although not shown in FIG. 1, a bonding agentlayer may be further formed on a lower side of the polarizer 110,thereby stacking the polarizing plate on a liquid crystal display panel.The bonding agent may include, but not limited to, a pressure-sensitivebonding agent.

Hereinafter, a polarizing plate according to another embodiment of thepresent invention will be described with reference to FIG. 2.

Referring to FIG. 2, a polarizing plate 200 according to anotherembodiment of the invention may include a polarizer 110; a polyesterfilm 120 formed on an upper side of the polarizer 110; and an opticalfilm 130 formed on a lower side of the polarizer 110. The polyester film120 may have a maximum thermal shrink angle of about 10° or less, andany one of nx and ny of about 1.65 or more. Except further including theoptical film 130, the polarizing plate 200 is substantially identical tothe polarizing plate 100 according to the first embodiment of thepresent invention. In this way, the polarizing plate may further includethe optical film, thereby enhancing mechanical strength thereof andminimizing the influence of the adhesive layers on the polarizer. Inthis regard, the optical film will hereinafter be described.

The optical film 130 is formed on one side of the liquid crystal displaypanel and may have a predetermined range of phase difference, therebycompensating for viewing angle. In one embodiment, the optical film mayhave a front retardation (Ro) of about 40 to about 60 nm at a wavelengthof 550 nm. Within this range, the optical film may exhibit an optimalimage quality. The optical film 130 may have a thickness of about 25 μmto about 500 μm, specifically about 25 to about 50 μm. Within thisrange, the optical film may be used to a polarizing plate for a liquidcrystal display apparatus.

The optical film 130 is a transparent optical film, and may be apolyester film including polyethylene naphthalate, polybutylenenaphthalate and the like, or a non-polyester film. Examples of thenon-polyester film may include celluloses including triacetyl celluloseand the like, cyclic polyolefin resins, polycarbonate resins; polyethersulfone resins; polysulfone resins, polyamide resins, polyimide resins,polyolefin resins, polyarcylate resins, polyvinyl alcohol resins,polyvinyl chloride resins, and polyvinylidene chloride resins.

Although not shown in FIG. 2, an adhesive layer for the polarizing platemay be formed between the polarizer 110 and the optical film 130 toimprove mechanical strength of the polarizing plate. The adhesive layermay include any typical adhesives, for example, at least one ofwater-based adhesive, pressure-sensitive adhesive, and photocurableadhesive. In addition, although not shown in FIG. 2, a bonding agentlayer for the polarizing plate may be further formed on a lower side ofthe optical film 130, thereby stacking the polarizing plate on a liquidcrystal display panel. The bonding agent may include, but not limitedto, a pressure-sensitive bonding agent.

Hereinafter, a method of preparing a polarizing plate according to thepresent invention will be described.

A method of preparing a polarizing plate according to one embodiment ofthe present invention may include drawing a melt-extruded polyesterresin in a draw ratio of about 2 to about 10, in TD only,thermal-stabilizing the drawn polyester resin to prepare a polyesterfilm, and bonding the polyester film to one side of a polarizer.

The polyester film may be prepared by drawing in TD alone withoutdrawing in MD. Thus, the polyester film may have TD draw ratio of about2 to about 10, and MD draw ratio of about 1 to about 1.1. Here, “MD drawratio of about 1 to about 1.1” means a state that there is no additionaldrawing process except inevitably being drawn due to mechanical movementof the film when drawing the film while moving it in MD. In particular,‘draw ratio of 1’ means a non-drawing state. The term ‘draw ratio’ asused in the polyester film may mean the ratio of a length of the filmafter drawing to a length of the film before drawing. When the film isdrawn by a draw ratio of about 2 to about 10 in both MD and TD,respectively, molecular orientation angle (θr) thereof may exceed about5° or retardation may be too low, so that rainbow spots may begenerated.

If TD draw ratio is less than about 2, the polyester film has a lowretardation, so that the polyester film can suffer from rainbow spots inapplying to a liquid crystal display apparatus, and easily tear due todeterioration of physical properties. On the other hand, if TD drawratio is more than about 10, the polyester film can be broken in thedrawing process. For example, TD draw ratio may be about 3 to about 8.

Drawing may be performed using at least one of dry drawing and wetdrawing. The drawing temperature may be from about (Tg−20)° C. to about(Tg+50)° C. based on Tg of the polyester resin, specifically about 70°C. to about 150° C., more specifically from about 80° C. to about 130°C., still more specifically from about 90° C. to about 120° C. Withinthis range, the polyester film having super high retardant mentionabove, maximum thermal shrinkage of about 0.8% or less, and maximumthermal shrink angle of about 10° or less can be prepared.

The method may further include thermal-stabilizing the drawn polyesterresin after drawing the polyester film. The polyester film drawn by highdraw ratio tends to have a restoring force which restores the film intoits original state. The thermal-stabilizing step may control stress tothe restoring force of the polyester film, thereby maintain thethermal-stabilization of the film. As a result, the polyester filmhaving maximum thermal shrinkage of about 0.8% or less, maximum thermalshrink angle of about 10° or less and molecular orientation angle of toabout 5° or less can be prepared.

The thermal-stabilizing step may include heating the drawn polyesterfilm whiling fixing the both TD ends of the polyester film and movingthe film in MD. Herein, the polyester film is drawn by a relatively lowdraw ratio compared to that of drawing step. TD draw ratio may be morethan about 0 to about 3 or less, specifically about 0.1 to about 2, morespecifically about 0.1 to about 1. Fixing the film in TD is performedjust enough to prevent restoration of the film due to the high drawratio. There is no substantial TD drawing effect on the polyester film(tension-relaxation).

Heating in the thermal-stabilizing step may be carried out at about 100to about 300° C. Within the range, the polyester film having maximumthermal shrinkage of about 0.8% or less, maximum thermal shrink angle ofabout 10° or less and molecular orientation angle of about 5° or lesscan be prepared. Heating may be carried out for about 1 sec to about 2hours.

The polarizer may be prepared by dyeing a polyvinyl alcohol film withiodine or a dichroic dye, followed by drawing the film in a certaindirection. The method of performing such steps may be generally known tothose skilled in the art.

The polyester film may be bonded to the polarizer using a typicaladhesive. The adhesive may include at least one of water-basedadhesives, pressure-sensitive adhesives, and photocurable adhesives.

The preparation method according to one embodiment of the presentinvention may further include bonding an optical film to the other sideof the polarizer. An adhesive for bonding the optical film may includeat least one of water-based adhesives, pressure-sensitive adhesives, andphotocurable adhesives.

The liquid crystal display apparatus of the present invention mayinclude a module for a liquid crystal display apparatus comprising thepolarizing plate according to embodiments of the present invention.Referring to FIG. 3, a module 300 for a liquid crystal display apparatusaccording to one embodiment of the present invention may include aliquid crystal display panel 310; a first polarizing plate 320 formed onan upper side of the liquid crystal display panel 310; a backlight unit340 formed on a lower side of the liquid crystal display panel 310; anda second polarizing plate 330 formed on the lower side of the liquidcrystal display panel 310 and placed between the liquid crystal displaypanel 310 and the backlight unit 340, wherein the first polarizing plate320 may include the polarizing plate according to one embodiment of thepresent invention.

The liquid crystal display panel 310 may include a liquid crystal panelcomprising a liquid crystal cell layer sealed between a first substrateand a second substrate. In one embodiment, the first substrate may be acolor filter (CF) substrate (upper substrate), and the second substratemay be a thin film transistor (TFT) substrate (lower substrate).

The first substrate and second substrate may be the same or different,and be a glass substrate or a plastic substrate. the plastic substratemay include polyethylene terephthalate (PET), polycarbonate (PC),polyimide (PI), polyethylene naphthalate (PEN), polyether sulfone (PES),polyacrylate (PAR), and cycloolefin copolymer (COC) substrates, whichcan be applied to flexible displays, without being limited thereto. theliquid crystal cell layer may a liquid crystal layer including verticalalignment (VA) mode, in-place switching (IPS) mode, fringe fieldswitching (FFS) mode, and twisted nematic (TN) mode liquid crystals.

The second polarizing plate 330 may be a common polarizing plate and maybe, for example, a polarizing plate comprising a polyester film, whichis one having a maximum thermal shrink angle of less than about 10°, onehaving any one of a refractive index of x-axis direction (nx) at awavelength of 550 nm and a refractive index of y-axis direction (ny) ata wavelength of 550 nm of less than about 1.65, or one having anabsolute value of molecular orientation angle of more than about 5°.

FIG. 3 shows the case that a first polarizing plate is the polarizingplate according to the first embodiment of the present invention.However, a second polarizing plate may be the polarizing plate accordingto one embodiment of the present invention, or both the first and secondpolarizing plate may be the polarizing plates according to theembodiment of the present invention. Thus, all these cases may beincluded in the scope of the subject invention.

The first polarizing plate 320 and second polarizing plate 330 may beformed on one side of a liquid crystal display panel by a bonding agentlayer (not shown in FIG. 3), respectively. The bonding agent layer maybe any typical bonding agents, for example, pressure sensitive bondingagents

The backlight unit 340 is one conventionally used in the liquid crystaldisplay apparatus and may include a light source, waveguide plate,reflecting plate, diffusion plate, and etc.

MODE FOR INVENTION

Hereinafter, the construction and functionality of the present inventionwill be described in more detail with reference to preferred examples ofthe present invention. However, it should be noted that these examplesare provided for the preferred illustrations of the present invention,and should be not construed in any way as limiting the presentinvention.

Details of components used in Examples and Comparative Examples are asfollows:

(1) Material of Polarizer: Polyvinyl alcohol film (VF-PS6000, KURARAYCo., Ltd., Japan, thickness: 60 μm)

(2) Polyethylene terephthalate film: Polyethylene terephthalate filmhaving nx, ny, maximum thermal shrinkage, maximum thermal shrink angleand molecular orientation angle shown in Table 1.

(3) Optical film: Triacetyl cellulose film (KC4DR-1, FUJI Co., Ltd.,Japan, thickness: 40 μm).

EXAMPLES 1 TO 4

A polyvinyl alcohol film was drawn to a drawing ratio of 3 at 60° C.,iodine was adsorbed onto the polyvinyl alcohol film, followed by drawingto a drawing ratio of 2.5 in a boric acid solution of 40° C., therebypreparing a polarizer. Then, a triacetyl cellulose film was stacked onone side of the polarizer using an adhesive (Z-200, NIPPON GOSHEI Co.,Ltd.), and a polyethylene terephthalate film shown in Table 1 wasstacked on the other side of the polarizer using an adhesive (Z-200,NIPPON GOSHEI Co., Ltd.) thereby preparing a polarizing plate.

Polyethylene terephthalate film shown in Table 1 was prepared bymelt-extruding a polyethylene terephthalate resin, drawing themelt-extruded film to a draw ratio of 6.1 in TD but not in MD whilemechanically moving the film in MD using a roll, under the conditionslisted in Table 1, followed by tension-relaxation treatment. Thepolyethylene terephthalate film had a thickness of 80 μm.

The maximum thermal shrinkage and maximum thermal shrink angle of thepolyethylene terephthalate film were measured using IM-6600 (KEYENCECo., Ltd.) according to FIG. 4. Here, a square polyester film specimenhaving a 200 mm length in MD and a 200 mm length in TD is used as a filmspecimen. The maximum thermal shrinkage and maximum thermal shrink anglewere measured on the specimen by drawing a circle (radius: 100 mm,having its center aligned with the center of the specimen), equallydividing a circumference of the circle to obtain a of 5°, and thenleaving the divided circle at 100° C. for 30 minutes. Ro of thepolyethylene terephthalate film was measured at a wavelength of 550 nmusing AXOSCAN (Axometrics Co., Ltd). The molecular orientation angle ofthe polyethylene terephthalate film was measured using KOBRA-WX100 (OjiCo., Ltd) and AXOSCAN (Axometrics Co., Ltd).

COMPARATIVE EXAMPLES 1 TO 3

A polarizing plate was prepared in the same manner as in Example 1except that polyethylene terephthalate films having nx, maximum thermalshrinkage, maximum thermal shrinkage, and molecular orientation anglelisted in Table 1 were used. The polyethylene terephthalate film inTable 1 was prepared by melt-extruding a polyethylene terephthalateresin, drawing the extruded resin under the conditions listed in Table1, followed by crystallization and stabilization treatments.

The films in Comparative Examples 1 to 2 were drawn to a ratio of 6.1 inTD but not in MD. The film in Comparative Example 3 was drawn in to adraw ratio of 3 in both MD and TD, respectively. The polyethyleneterephthalate film had a thickness of 80 μm.

TABLE 1 Maximum thermal Maximum Molecular Tension-Relaxation shrinkthermal orientation Temp. thickness angle shrinkage angle Ro Treatment(° C.) nx ny (μm) (°) (%) (°) (nm) Example 1 ◯ 100 1.69 1.56 80 9 0.582.1 10,100 Example 2 ◯ 150 1.69 1.56 80 7 0.55 2.1 10,100 Example 3 ◯200 1.69 1.56 80 5 0.43 2.1 10,100 Example 4 ◯ 250 1.69 1.56 80 2 0.210.0 10,100 Comparative X — 1.69 1.56 80 16 1.02 9.7 10,100 Example 1Comparative ◯  70 1.64 1.58 80 7 1.02 31.4 4,800 Example 2 Comparative ◯100 1.64 1.63 80 9 0.58 43.6 800 Example 3

The polarizing plates prepared in Examples and Comparative Examples wereevaluated as to the following properties. Results are shown in Table 2.

TABLE 2 Ttransmittance (Ts, %) Polarization degree After left (PE, %)Rainbow spots for a long After left for After left for period of a longperiod a long period Contrast time at high of time at of time at CR/CROInitial temp. Initial high temp. Initial high temp CR (%) Example 142.60 42.69 99.9931 99.9925 X X 5,340 1.40 Example 2 42.60 42.69 99.994599.9939 X X 5,450 1.43 Example 3 42.60 42.68 99.9965 99.9959 X X 5,5801.46 Example 4 42.60 42.69 99.9985 99.9979 X X 5,870 1.54 Comparative42.60 42.67 99.9402 99.9382 X X 3,810 1.00 Example 1 Comparative 42.6042.68 99.9894 99.9878 ◯ ◯ 4,869 1.28 Example 2 Comparative 42.60 42.6899.9945 99.9939 ◯ ◯ 5,450 1.43 Example 3

(1) Transmittance and Polarization Degree: Transmittance andpolarization degree for a polarizing plate were measured using V7100(JASCO Co., Ltd.). Further, after left the polarizing plate at hightemperature for a long period of time (e.g., at 85° C. for 120 hours),transmittance and polarization degree were evaluated in the same methodas that mentioned above. Herein, the measurement was performed atwavelength 550 nm.

(2) Rainbow spots: The polarizing plates were displaced on an upper sideof a liquid crystal display panel, on a lower side of a liquid crystaldisplay panel of a VA mode liquid crystal, and between a liquid crystaldisplay panel and a backlight unit, respectively, to assemble them.Using a Spectroradiometer (SR-3A, TOPCON Co., Ltd.), it was observedwhether rainbow spots were generated. When there are no rainbow spots,it was evaluated as X, and when there are rainbow spots, it wasevaluated as ◯. In addition, after leaving the polarizing plate at hightemperature for a long period of time (e.g., at 85° C. for 120 hours),it was evaluated whether rainbow spots were generated, in a mannersimilar to the above method.

(3) Contrast: The polarizing plates were displaced on the upper side ofthe liquid crystal display panel, on the lower side of the liquidcrystal display panel of VA mode liquid crystal (Model name) and betweenthe liquid crystal display panel and the backlight unit, respectively,to assemble them, followed by measuring Contrast Ratio (CR) using aluminance meter SR-3A (Topcon Co., Ltd.). Also, a contrast ratio of thespecimen of Comparative Example 1 (which has the lowest contrast ratio)was taken as CRO. Thus, Contrast ratio (CR/CRO) was calculated as apercent ratio of CR to CRO.

As shown in Table 2, the polarizing plate of the present invention didnot suffer from rainbow spots, and optical properties includingpolarization degree and transmittance were good. Moreover, even afterleft at high temperature for a long period of time, the polarizing platedid not suffer from rainbow spots, and optical properties includingpolarization degree and transmittance were not changed.

On the contrary, in Comparative Example 1, the film was drawn to a drawratio of 6.1 in TD, did not tension-relaxed, and had a maximum thermalshrink angle of more than 10°. Thus, Comparative Example 1 did notsuffer from rainbow spots, but had bad polarization degree. Stillfurther, even after left at high temperature for a long period of time,polarization degree and transmittance of the polarizing plate inComparative Example 1 were decreased in comparison with the presentinvention.

Further, in Comparative Example 2, the film was drawn to a draw ratio of6.1 in TD, but a temperature range in tension-relaxing got out of therange of the present invention, and any one of nx and ny thereof was not1.65 or more. Thus, Comparative Example 2 suffered from rainbow spots,and had an inferior polarization degree. Even after left at hightemperature for a long period of time, the polarizing plate inComparative Example 2 had an inferior polarization degree in comparisonwith the present invention.

Still further, in Comparative Example 3, the film was drawn to a drawratio of 3 in both MD and TD, and any one of nx and ny thereof was not1.65 or more. Thus, Comparative Example 3 suffered from rainbow spots.Moreover, even after left at high temperature for a long period of time,the polarizing plate in Comparative Example 3 still suffered fromrainbow spots.

Simple modification or changes of the present invention can be easilyperformed by those of ordinary skill in the art. Therefore, it can beregarded that such modifications or changes are included in the scope ofthe present invention.

The invention claimed is:
 1. A polarizing plate comprising: a polarizer;and a polyester film formed on an upper side of the polarizer, whereinthe polyester film has a maximum thermal shrink angle of about 10° orless, a refractive index of an x-axis direction (nx) at a wavelength of550 nm of about 1.65 or more, a refractive index of a y-axis direction(ny) at a wavelength of 550 nm of about 1.65 or less, and a frontretardation (Ro) of about 5,000 to about 15,000 nm at a wavelength of550 nm, wherein the polyester film has a retardation in a thicknessdirection (Rth) of 10,000 nm to 13,000 nm at a wavelength of 550 nm, ascalculated by Equation 2:Rth={(nx+ny)/2−nz}×d  Equation 2 wherein, in Equation 2, nx, ny and nzare refractive indices in an x-axis direction, a y-axis direction and az-axis direction of the polyester film at a wavelength of 550 nm,respectively, and d is a thickness of the film in nm.
 2. The polarizingplate according to claim 1, wherein an absolute value of nx−ny is about0.1 to about 0.2.
 3. The polarizing plate according to claim 1, whereinthe polyester film has a maximum thermal shrinkage of about 0.8% orless.
 4. The polarizing plate according to claim 1, wherein thepolyester film has an absolute value of a molecular orientation angle(θr) of a polyester molecule based on a transverse direction (TD) ofabout 0° to about 5°.
 5. The polarizing plate according to claim 1,wherein the polyester film has a thickness of about 25 to about 115 μm.6. The polarizing plate according to claim 1, wherein the frontretardation (Ro) of the polyester film is about 10,100 to about 12,000nm.
 7. The polarizing plate according to claim 1, wherein the polyesterfilm is a TD drawn film.
 8. The polarizing plate according to claim 1,wherein the polyester film has a degree of biaxiality (NZ) of about 1.8or less at a wavelength of 550 nm, as represented by Equation 1:NZ=(nx−nz)/(nx−ny)  Equation 1 (wherein nx, ny and nz are refractiveindices in an x-axis direction, a y-axis direction and a z-axisdirection of the polyester film at a wavelength of 550 nm,respectively).
 9. The polarizing plate according to claim 1, wherein thepolyester film is a film formed of at least one of polyethyleneterephthalate, polybutylene terephthalate, polyethylene naphthalate andpolybutylene naphthalate.
 10. The polarizing plate according to claim 1,further comprising an optical film on a lower side of the polarizer. 11.The polarizing plate according to claim 10, wherein the optical film hasa front retardation (Ro) of about 40 to about 60 nm at a wavelength of550 nm.
 12. The polarizing plate according to claim 10, wherein theoptical film is a film formed of at least one of cellulose, polyester,cyclic polyolefin, polycarbonate, polyether sulfone, polysulfone,polyimide, polyimide, polyolefin, polyacrylate, polyvinyl alcohol,polyvinyl chloride, and polyvinylidene chloride resins.
 13. Thepolarizing plate according to claim 1, wherein the polarizing plate hasa polarization degree of about 99.99% or more, and a transmittance ofabout 40% or more.
 14. A liquid crystal display apparatus comprising thepolarizing plate according to claim
 1. 15. A polarizing platecomprising: a polarizer; and a polyester film formed on an upper side ofthe polarizer, wherein the polyester film has an absolute value of amolecular orientation angle (θr) of about 5° or less, and wherein thepolyester film has a refractive index of an x-axis direction (nx) at awavelength of 550 nm of about 1.65 or more, a refractive index of ay-axis direction (ny) at a wavelength of 550 nm of about 1.65 or less,and a front retardation (Ro) of about 5,000 to about 15,000 nm at awavelength of 550 nm, wherein the polyester film has a retardation in athickness direction (Rth) of 10,000 nm to 13,000 nm at a wavelength of550 nm, as calculated by Equation 2:Rth={(nx+ny)/2−nz}×d  Equation 2 wherein, in Equation 2, nx, ny and nzare refractive indices in an x-axis direction, a y-axis direction and az-axis direction of the polyester film at a wavelength of 550 nm,respectively, and d is a thickness of the film in nm.